| CoRE | Z. Shelby |
| Internet-Draft | Sensinode |
| Intended status: Standards Track | S. Krco |
| Expires: December 05, 2013 | Ericsson |
| C. Bormann | |
| Universitaet Bremen TZI | |
| June 03, 2013 |
CoRE Resource Directory
draft-ietf-core-resource-directory-00
In many M2M applications, direct discovery of resources is not practical due to sleeping nodes, disperse networks, or networks where multicast traffic is inefficient. These problems can be solved by employing an entity called a Resource Directory (RD), which hosts descriptions of resources held on other servers, allowing lookups to be performed for those resources. This document specifies the web interfaces that a Resource Directory supports in order for web servers to discover the RD and to register, maintain, lookup and remove resources descriptions. Furthermore, new link attributes useful in conjunction with an RD are defined.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 05, 2013.
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The Constrained RESTful Environments (CoRE) work aims at realizing the REST architecture in a suitable form for the most constrained nodes (e.g. 8-bit microcontrollers with limited RAM and ROM) and networks (e.g. 6LoWPAN). CoRE is aimed at machine-to-machine (M2M) applications such as smart energy and building automation.
The discovery of resources offered by a constrained server is very important in machine-to-machine applications where there are no humans in the loop and static interfaces result in fragility. The discovery of resources provided by an HTTP Web Server is typically called Web Linking [RFC5988]. The use of Web Linking for the description and discovery of resources hosted by constrained web servers is specified by the CoRE Link Format [RFC6690]. This specification however only describes how to discover resources from the web server that hosts them by requesting /.well-known/core. In many M2M scenarios, direct discovery of resources is not practical due to sleeping nodes, disperse networks, or networks where multicast traffic is inefficient. These problems can be solved by employing an entity called a Resource Directory (RD), which hosts descriptions of resources held on other servers, allowing lookups to be performed for those resources.
This document specifies the web interfaces that a Resource Directory supports in order for web servers to discover the RD and to register, maintain, lookup and remove resource descriptions. Furthermore, new link attributes useful in conjunction with a Resource Directory are defined. Although the examples in this document show the use of these interfaces with CoAP [I-D.ietf-core-coap], they may be applied in an equivalent manner to HTTP [RFC2616].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. The term "byte" is used in its now customary sense as a synonym for "octet".
This specification requires readers to be familiar with all the terms and concepts that are discussed in [RFC5988] and [RFC6690]. Readers should also be familiar with the terms and concepts discussed in [I-D.ietf-core-coap]. The URI Template format is used to describe the REST interfaces defined in this specification [RFC6570]. This specification makes use of the following additional terminology:
The resource directory architecture is shown in Figure 1. A Resource Directory (RD) is used as a repository for Web Links [RFC5988] about resources hosted on other web servers, which are called endpoints (EP). An endpoint is a web server associated with an IP address and port, thus a physical node may host one or more endpoints. The RD implements a set of REST interfaces for endpoints to register and maintain sets of Web Links (called resource directory entries), for the RD to validate entries, and for clients to lookup resources from the RD. Endpoints themselves can also act as clients. An RD can be logically segmented by the use of Domains. The domain an endpoint is associated with can be defined by the RD or configured by an outside entity.
Endpoints are assumed to proactively register and maintain resource directory entries on the RD, which are soft state and need to be periodically refreshed. An endpoint is provided with interfaces to register, update and remove a resource directory entry. Furthermore, a mechanism to discover a RD using the CoRE Link Format is defined. It is also possible for an RD to proactively discover Web Links from endpoints and add them as resource directory entries, or to validate existing resource directory entries. A lookup interface for discovering any of the Web Links held in the RD is provided using the CoRE Link Format.
Registration Lookup
+----+ | |
| EP |---- | |
+----+ ---- | |
--|- +------+ |
+----+ | ----| | | +--------+
| EP | ---------|-----| RD |----|-----| Client |
+----+ | ----| | | +--------+
--|- +------+ |
+----+ ---- | |
| EP |---- | |
+----+
Figure 1: The resource directory architecture.
Over the last few years, mobile operators around the world have focused on development of M2M solutions in order to expand the business to the new type of users, i.e. machines. The machines are connected directly to a mobile network using appropriate embedded air interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing short and wide range wireless interfaces. From the system design point of view, the ambition is to design horizontal solutions that can enable utilization of machines in different applications depending on their current availability and capabilities as well as application requirements, thus avoiding silo like solutions. One of the crucial enablers of such design is the ability to discover resources (machines - endpoints) capable of providing required information at a given time or acting on instructions from the end users.
In a typical scenario, during a boot-up procedure (and periodically afterwards), the machines (endpoints) register with a Resource Directory (for example EPs installed on vehicles enabling tracking of their position for the fleet management purposes and monitoring environment parameters) hosted by the mobile operator or somewhere else in the network, periodically a description of its own capabilities. Due to the usual network configuration of mobile networks, the EPs attached to the mobile network do not have routable addresses. Therefore, a remote server is usually used to provide proxy access to the EPs. The address of each (proxy) endpoint on this server is included in the resource description stored in the RD. The users, for example mobile applications for environment monitoring, contact the RD, look-up the endpoints capable of providing information about the environment using appropriate set of link parameters, obtain information on how to contact them (URLs of the proxy server) and then initiate interaction to obtain information that is finally processed, displayed on the screen and usually stored in a database. Similarly, fleet management systems provide the appropriate link parameters to the RD to look-up for EPs deployed on the vehicles the application is responsible for.
Home and commercial building automation systems can benefit from the use of M2M web services. The use of CoRE in home automation across multiple subnets is described in [I-D.brandt-coap-subnet-discovery] and in commercial building automation in [I-D.vanderstok-core-bc]. The discovery requirements of these applications are demanding. Home automation usually relies on run-time discovery to commission the system, whereas in building automation a combination of professional commissioning and run-time discovery is used. Both home and building automation involve peer-to-peer interactions between endpoints, and involve battery-powered sleeping devices.
The exporting of resource information to other discovery systems is also important in these automation applications. In home automation there is a need to interact with other consumer electronics, which may already support DNS-SD, and in building automation larger resource directories or DNS-SD covering multiple buildings.
Not all endpoints hosting resources are expected to know how to implement the Resource Directory Function Set and thus explicitly register with a Resource Directory (or other such directory server). Instead, simple endpoints can implement the generic Simple Directory Discovery approach described in this section. An RD implementing this specification MUST implement Simple Directory Discovery. However, there may be security reasons why this form of directory discovery would be disabled.
This approach requires that the endpoint makes available the hosted resources that it wants to be discovered, as links on its /.well-known/core interface as specified in [RFC6690].
The endpoint then finds one or more IP addresses of the directory server it wants to know about its resources as described in Section 4.1.
An endpoint that wants to make itself discoverable occasionally sends a POST request to the /.well-known/core URI of any candidate directory server that it finds. The body of the POST request is either
or
The directory server integrates the information it received this way into its resource directory. It MAY make the information available to further directories, if it can ensure that a loop does not form. The protocol used between directories to ensure loop-free operation is outside the scope of this document.
The following example shows an endpoint using simple resource discovery, by simply sending a POST with its links in the body to a directory.
EP RD
| |
| -- POST /.well-known/core "</sen/temp>..." ---> |
| |
| |
| <---- 2.01 Created ------------------------- |
| |
Endpoints that want to contact a directory server can obtain candidate IP addresses for such servers in a number of ways.
In a 6LoWPAN, good candidates can be taken from:
In networks with more inexpensive use of multicast, the candidate IP address may be a well-known multicast address, i.e. directory servers are found by simply sending POST requests to that well-known multicast address (details TBD).
As some of these sources are just (more or less educated) guesses, endpoints MUST make use of any error messages to very strictly rate-limit requests to candidate IP addresses that don't work out. E.g., an ICMP Destination Unreachable message (and, in particular, the port unreachable code for this message) may indicate the lack of a CoAP server on the candidate host, or a CoAP error response code such as 4.05 "Method Not Allowed" may indicate unwillingness of a CoAP server to act as a directory server.
This section defines the REST interfaces between an RD and endpoint servers, which is called the Resource Directory Function Set. Although the examples throughout this section assume use of CoAP [I-D.ietf-core-coap], these REST interfaces can also be realized using HTTP [RFC2616]. An RD implementing this specification MUST support the discovery, registration, update, and removal interfaces defined in this section and MAY support the validation interface. For the purpose of validation, an endpoint implementing this specification SHOULD support ETag validation on /.well-known/core (which is very straightforward for static /.well-known/core link documents).
Resource directory entries are designed to be easily exported to other discovery mechanisms such as DNS-SD. For that reason, parameters that would meaningfully be mapped to DNS are limited to a maximum length of 63 bytes.
Before an endpoint can make use of an RD, it must first know the RD's IP address, port and the path of its RD Function Set. There can be several mechanisms for discovering the RD including assuming a default location (e.g. on an Edge Router in a LoWPAN), by assigning an anycast address to the RD, using DHCP, or by discovering the RD using the CoRE Link Format (also see Section 4.1). This section defines discovery of the RD using the well-known interface of the CoRE Link Format [RFC6690] as the required mechanism. It is however expected that RDs will also be discoverable via other methods depending on the deployment.
Discovery is performed by sending either a multicast or unicast GET request to /.well-known/core and including a Resource Type (rt) parameter [RFC6690] with the value "core.rd" in the query string. Likewise, a Resource Type parameter value of "core.rd-lookup" is used to discover the RD Lookup Function Set. Upon success, the response will contain a payload with a link format entry for each RD discovered, with the URL indicating the root resource of the RD. When performing multicast discovery, the multicast IP address used will depend on the scope required and the multicast capabilities of the network.
An RD implementation of this specification MUST support query filtering for the rt parameter as defined in [RFC6690].
The discovery request interface is specified as follows:
The following response codes are defined for this interface:
The following example shows an endpoint discovering an RD using this interface, thus learning that the base RD resource is at /rd. Note that it is up to the RD to choose its base RD resource, although it is recommended to use the base paths specified here where possible.
EP RD
| |
| ----- GET /.well-known/core?rt=core.rd* ------> |
| |
| |
| <---- 2.05 Content "</rd>; rt="core.rd" ------ |
| |
Req: GET coap://[ff02::1]/.well-known/core?rt=core.rd*
Res: 2.05 Content
</rd>;rt="core.rd",
</rd-lookup>;rt="core.rd-lookup",
</rd-group>;rt="core.rd-group"
After discovering the location of an RD Function Set, an endpoint MAY register its resources using the registration interface. This interface accepts a POST from an endpoint containing the list of resources to be added to the directory as the message payload in the CoRE Link Format along with query string parameters indicating the name of the endpoint, its domain and the lifetime of the registration. All parameters except the endpoint name are optional. It is expected that other specifications MAY define further parameters (it is to be determined if a registry of parameters is needed for this purpose). The RD then creates a new resource or updates an existing resource in the RD and returns its location. An endpoint MUST use that location when refreshing registrations using this interface. Endpoint resources in the RD are kept active for the period indicated by the lifetime parameter. The endpoint is responsible for refreshing the entry within this period using either the registration or update interface. The registration interface MUST be implemented to be idempotent, so that registering twice with the same endpoint parameter does not create multiple RD entries.
The registration request interface is specified as follows:
The following response codes are defined for this interface:
The following example shows an endpoint with the name "node1" registering two resources to an RD using this interface. The resulting location /rd/4521 is just an example of an RD generated location.
EP RD
| |
| --- POST /rd?ep=node1 "</sensors..." -------> |
| |
| |
| <-- 2.01 Created Location: /rd/4521 ---------- |
| |
Req: POST coap://rd.example.com/rd?ep=node1
Payload:
</sensors/temp>;ct=41;rt="temperature-c";if="sensor",
</sensors/light>;ct=41;rt="light-lux";if="sensor"
Res: 2.01 Created
Location: /rd/4521
The update interface is used by an endpoint to refresh or update its registration with an RD. To use the interface, the endpoint sends a PUT request to the resource returned in the Location option in the response to the first registration. An update MAY contain registration parameters if there have been changes since the last registration or update. Parameters that have not changed SHOULD NOT be included in an update. Upon receiving an update request, the RD resets the timeout for that endpoint and stores the values of the parameters included in the update (if any).
The update request interface is specified as follows:
The following response codes are defined for this interface:
The following example shows an endpoint updating a new set of resources to an RD using this interface.
EP RD
| |
| --- PUT /rd/4521 --------------------------> |
| |
| |
| <-- 2.04 Changed ---------------------------- |
| |
Req: PUT /rd/4521
Res: 2.04 Changed
In some cases, an RD may want to validate that it has the latest version of an endpoint's resources. This can be performed with a GET on the well-known interface of the CoRE Link Format including the latest ETag stored for that endpoint. For the purpose of validation, an endpoint implementing this specification SHOULD support ETag validation on /.well-known/core.
The validation request interface is specified as follows:
The following responses codes are defined for this interface:
The following examples shows a successful validation.
EP RD
| |
| <--- GET /.well-known/core ETag: 0x40 -------- |
| |
| |
| --- 2.03 Valid -----------------------------> |
| |
Req: GET /.well-known/core
ETag: 0x40
Res: 2.03 Valid
Although RD entries have soft state and will eventually timeout after their lifetime, an endpoint SHOULD explicitly remove its entry from the RD if it knows it will no longer be available (for example on shut-down). This is accomplished using a removal interface on the RD by performing a DELETE on the endpoint resource.
The removal request interface is specified as follows:
The following responses codes are defined for this interface:
The following examples shows successful removal of the endpoint from the RD.
EP RD
| |
| --- DELETE /rd/4521 ------------------------> |
| |
| |
| <-- 2.02 Deleted ---------------------------- |
| |
Req: DELETE /rd/4521
Res: 2.02 Deleted
This section defines a function set for the creation of groups of endpoints for the purpose of managing and looking up endpoints for group operations. The group function set is similar to the resource directory function set, in that a group may be created or removed. However unlike an endpoint entry, a group entry consists of a list of endpoints and does not have a lifetime associated with it. In order to make use of multicast requests with CoAP, a group MAY have a multicast address associated with it.
In order to create a group, a management entity used to configure groups, makes a request to the RD indicating the name of the group to create (or update), the optional domain the group belongs to, and the optional multicast address of the group. The registration message includes the list of endpoints that belong to that group. If an endpoint has already registered with the RD, the RD attempts to use the context of the endpoint from its RD endpoint entry. If the client registering the group knows the endpoint has already registered, then it MAY send a blank target URI for that endpoint link when registering the group.
The registration request interface is specified as follows:
The following response codes are defined for this interface:
The following example shows a group with the name "lights" registering two endpoints to an RD using this interface. The resulting location /rd-group/12 is just an example of an RD generated group location.
EP RD
| |
| - POST /rd-group?gp=lights "<>;ep=node1..." --> |
| |
| |
| <---- 2.01 Created Location: /rd-group/12 ---- |
| |
Req: POST coap://rd.example.com/rd-group?gp=lights
Payload:
<>;ep="node1",
<>;ep="node2"
Res: 2.01 Created
Location: /rd-group/12
A group can be removed simply by sending a removal message to the location returned when registering the group. Removing a group MUST NOT remove the endpoints of the group from the RD.
The removal request interface is specified as follows:
The following responses codes are defined for this interface:
The following examples shows successful removal of the group from the RD.
EP RD
| |
| --- DELETE /rd-group/412 -------------------> |
| |
| |
| <-- 2.02 Deleted ---------------------------- |
| |
Req: DELETE /rd-group/12
Res: 2.02 Deleted
In order for an RD to be used for discovering resources registered with it, a lookup interface can be provided using this function set. This lookup interface is defined as a default, and it is assumed that RDs may also support lookups to return resource descriptions in alternative formats (e.g. Atom or HTML Link) or using more advanced interfaces (e.g. supporting context or semantic based lookup).
This function set allows lookups for domains, groups, endpoints and resources using attributes defined in the RD Function Set and for use with the CoRE Link Format. The result of a lookup request is the list of links (if any) in CoRE Link Format corresponding to the type of lookup. The target of these links SHOULD be the actual location of the domain, endpoint or resource, but MAY be an intermediate proxy e.g. in the case of an HTTP lookup interface for CoAP endpoints. Multiple query parameters MAY be included in a lookup, all included parameters MUST match for a resource to be returned. The character '*' MAY be included at the end of a parameter value as a wildcard operator.
The lookup interface is specified as follows:
The following responses codes are defined for this interface:
The following example shows a client performing a resource lookup:
Client RD
| |
| ----- GET /rd-lookup/res?rt=temperature -----------------> |
| |
| |
| <-- 2.05 Content "<coap://node1/temp>;rt="temperature" ---- |
| |
Req: GET /rd-lookup/res?rt=temperature
Res: 2.05 Content
<coap://{ip:port}/temp>
The following example shows a client performing an endpoint lookup:
Client RD
| |
| ----- GET /rd-lookup/ep?et=power-node --------------------> |
| |
| |
| <-- 2.05 Content "<coap://{ip:port}>;ep="node5" ----------- |
| |
Req: GET /rd-lookup/ep?et=power-node
Res: 2.05 Content
<coap://{ip:port}>;ep="node5",
<coap://{ip:port}>;ep="node7"
The following example shows a client performing a domain lookup:
Client RD
| |
| ----- GET /rd-lookup/d ----------------------------------> |
| |
| |
| <-- 2.05 Content "</rd>;d=domain1,</rd>;d=domain2 --------- |
| |
Req: GET /rd-lookup/d
Res: 2.05 Content
</rd>;d="domain1",
</rd>;d="domain2"
The following example shows a client performing a group lookup for all groups:
Client RD
| |
| ----- GET /rd-lookup/gp ---------------------------------> |
| |
| |
| <-- 2.05 Content </rd-group/12>;gp="lights1";d="domain1" -- |
| |
Req: GET /rd-lookup/gp
Res: 2.05 Content
</rd-group/12>;gp="lights1";d="domain1"
The following example shows a client performing a lookup for all endpoints in a particular group:
Client RD
| |
| ----- GET GET /rd-lookup/ep?gp=lights1-------------------> |
| |
| |
| <-- 2.05 Content "</rd>;d=domain1,</rd>;d=domain2 --------- |
| |
Req: GET /rd-lookup/ep?gp=lights1
Res: 2.05 Content
<coap://host:port>;ep="node1",
<coap://host:port>;ep="node2",
The following example shows a client performing a lookup for all groups an endpoint belongs to:
Client RD
| |
| ----- GET /rd-lookup/gp?ep=node1 ------------------------> |
| |
| |
| <-- 2.05 Content "</rd>;d=domain1,</rd>;d=domain2 --------- |
| |
Req: GET /rd-lookup/gp?ep=node1
Res: 2.05 Content
<coap://host:port>;gp="lights1";ep="node1",
When using the CoRE Link Format to describe resources being discovered by or posted to a resource directory service, additional information about those resources is useful. This specification defines the following new attributes for use in the CoRE Link Format [RFC6690]:
link-extension = ( "ins" "=" quoted-string ) ; Max 63 bytes
link-extension = ( "exp" )
The Resource Instance "ins" attribute is an identifier for this resource, which makes it possible to distinguish from other similar resources. This attribute is similar in use to the "Instance" portion of a DNS-SD record, and SHOULD be unique across resources with the same Resource Type attribute in the domain it is used. A Resource Instance might be a descriptive string like "Ceiling Light, Room 3", a short ID like "AF39" or a unique UUID or iNumber. This attribute is used by a Resource Directory to distinguish between multiple instances of the same resource type within a system.
This attribute MUST be no more than 63 bytes in length. The resource identifier attribute MUST NOT appear more than once in a link description.
The Export "exp" attribute is used as a flag to indicate that a link description MAY be exported by a resource directory to external directories.
The CoRE Link Format is used for many purposes between CoAP endpoints. Some are useful mainly locally, for example checking the observability of a resource before accessing it, determining the size of a resource, or traversing dynamic resource structures. However, other links are very useful to be exported to other directories, for example the entry point resource to a functional service.
This document needs the same security considerations as described in Section 7 of [RFC5988] and Section 6 of [RFC6690]. The /.well-known/core resource may be protected e.g. using DTLS when hosted on a CoAP server as described in [I-D.ietf-core-coap].
Access control SHOULD be performed separately for the RD Function Set and the RD Lookup Function Set, as different endpoints may be authorized to register with an RD from those authorized to lookup endpoints from the RD. Such access control SHOULD be performed in as fine-grained a level as possible. For example access control for lookups could be performed either at the domain, endpoint or resource level.
"core.rd", "core.rd-group" and "core.rd-lookup" resource types need to be registered with the resource type registry defined by [RFC6690].
The "exp" attribute needs to be registered when a future Web Linking attribute is created.
Szymon Sasin, Kerry Lynn, Esko Dijk, Peter van der Stok, Anders Brandt, Matthieu Vial, Sampo Ukkola and Linyi Tian have provided helpful comments, discussions and ideas to improve and shape this document. The authors would also like to thank their collagues from the EU FP7 SENSEI project, where many of the resource directory concepts were originally developed.
Changes from -05 to WG Document -00:
Changes from -04 to -05:
Changes from -03 to -04:
Changes from -02 to -03:
Changes from -01 to -02:
| [RFC6690] | Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, August 2012. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC5988] | Nottingham, M., "Web Linking", RFC 5988, October 2010. |
| [RFC6570] | Gregorio, J., Fielding, R., Hadley, M., Nottingham, M. and D. Orchard, "URI Template", RFC 6570, March 2012. |
| [I-D.ietf-core-coap] | Shelby, Z., Hartke, K. and C. Bormann, "Constrained Application Protocol (CoAP)", Internet-Draft draft-ietf-core-coap-14, March 2013. |
| [I-D.vanderstok-core-bc] | Stok, P. and K. Lynn, "CoAP Utilization for Building Control", Internet-Draft draft-vanderstok-core-bc-05, October 2011. |
| [I-D.brandt-coap-subnet-discovery] | Brandt, A., "Discovery of CoAP servers across subnets", Internet-Draft draft-brandt-coap-subnet-discovery-00, March 2011. |
| [RFC6775] | Shelby, Z., Chakrabarti, S., Nordmark, E. and C. Bormann, "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC 6775, November 2012. |
| [RFC2616] | Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. |